Synopsis Inter-Canadien Flight 1628, an ATR 42-300 (Serial No 143), was cleared to take off from runway 06. This regularly scheduled passenger flight links Quebec City, Quebec, and Baie-Comeau, Quebec. During climb-out, the aircraft flew over a convoy of six snow removal vehicles travelling in the opposite direction at the midway point of the runway. The flight crew reported the incident to the air traffic controller, then continued the climb and flew on to the destination without further incident. Ce rapport est galement disponible en franais. Other Factual Information At 0819, eastern daylight time (EDT),(1) a convoy consisting of six snow removal vehicles under the direction of the maintenance supervisor (Staff 22) was authorized by the ground controller to proceed onto runway 06 via taxiway Bravo, runway 30 and taxiway Golf. At 0824, the convoy entered runway 06 and cleared it six minutes later via taxiway Alpha to allow an aircraft to land. After clearing the taxiway, Staff 22 reported to the ground controller that the vehicles were going to work on the apron and asked to be advised when the runway was available. At 0837, the aircraft landed and was cleared to leave the runway via taxiway Alpha and travel to the apron. About one minute and twenty-four seconds later, the ground controller advised Staff 22 that the runway was available. Shortly thereafter, the ground controller was relieved for a rest break, by the shift supervisor. Staff 22 and the five other vehicles then left the apron and entered runway 06 via taxiway Alpha without requesting permission to proceed. At 0841, Inter-Canadien 1628 (ICN 1628) was cleared by the ground controller to proceed onto runway 06. The investigation revealed that the communication was heard by two vehicle drivers while the convoy was on runway 06, about 1,000 feet from the intersection with taxiway Alpha. The drivers were under the impression that the controller would warn them if they had to clear the runway. Shortly thereafter, the convoy of vehicles turned around and headed towards the right side of runway 24. At 0845, the air controller cleared ICN 1628 to take off from runway 06. The co-pilot was at the aircraft's controls during the take-off run. At rotation speed, he noticed that there were vehicles on the runway and pitched the nose of the aircraft up more steeply than normal. Shortly thereafter, the captain reported to the air controller that there were several vehicles on the runway. In the meantime, Staff 22 advised the ground controller that an aircraft had just flown over the vehicles. The aircraft was estimated to have taken off approximately 3,000 feet from the threshold of runway 06 between 1, 000 and 1,500 feet before the area where the vehicles were. The climbing aircraft flew over the vehicles at an altitude of between 200 and 300 feet. Runway 06/24 is 9,000 feet long and 150 feet wide. The weather conditions prevailing at Quebec City at the time of the occurrence were as follows: surface winds from 090 degrees magnetic at 15 knots gusting to 20 knots; visibility half mile in average-intensity and blowing snow; vertical visibility 600 feet; temperature minus 7 degrees Celsius; dew point minus 8 degrees Celsius; barometric pressure 29.36 inches of mercury; obscuring phenomenon due to snow; opacity 8/10. The runway and the apron could not be seen from the control tower because of the weather conditions. The frequency 121.9 megahertz was used for communications between the control tower personnel and the snow removal vehicles. Use of the ground control frequency for communications with all airport vehicles is the established standard procedure. The maintenance frequency was used for communications between the snow removal personnel. This frequency is also used by all airport vehicles, but is not available to air traffic personnel. It is a work tool and helps to reduce congestion on the ground frequency. Staff 22 stated that he did not hear the ground controller clear ICN 1628 to proceed because the maintenance frequency was congested. The communication equipment was working properly at the time of the occurrence. Controllers occupied the ground and air positions in the control tower, and there was also a shift supervisor present. Staffing met unit standards. The transfer of responsibilities between the supervisor and the ground controller for his rest break was done in accordance with standard procedures. The supervisor knew that Staff 22 had been advised that the runway was available, and he was under the impression that the snow removal vehicles would continue to work on the apron. The workload was assessed as light, with normal complexity. The personnel on duty in the control tower and the snow removal personnel were certified and qualified. In the tower, red and green warning lights report movements and obstacles on the runways. The ground controller attends to the warning lights. A green light indicates that the runway is clear, and a red light indicates that the runway cannot be used. In the present case, the green warning light for runway 06 was lit. In addition to these lights, vehicle information is recorded and updated on red progress strips showing the number and position of vehicles and placed directly under the flight progress strips. In the present case, there was a red strip on the ground and air controllers' data blocks. At the time of the occurrence, each strip indicated that the six snow removal vehicles were on the apron. The airport is not equipped with Airport Surface Detection Equipment (ASDE). An ASDE is a very useful tool for navigation on the ground, as it can detect potential conflict situations, especially if visibility is restricted. In performing their duties, the air traffic and maintenance personnel were using the Snow Removal Manual 1996-1997, Jean-Lesage International Airport, 23 October 1996 and the minutes of a snow removal meeting held on 28 October 1996. At this meeting it was agreed, among other things, that, to allow good cooperation during storms and to speed up snow removal work, the head of maintenance was to be advised as soon as the active runway was available, and that care was to be taken to confirm whether authorization had been given to proceed along the whole length of the runway or with restriction. The ground controller concerned, and the maintenance supervisor had attended this meeting, and the maintenance supervisor had prepared and signed the agreed procedure. In the present case, Staff 22 did not see the need to request permission to proceed because the runway that he had just cleared was available again. Further, he thought that he could use its whole length without restriction. The Air Traffic Control Manual of Operations (MANOPS) and the Airport Traffic Directives for the Operation of Vehicles on Airport Movement Areas (TP 2633) deal with ground traffic at airports and contain the standard phraseology to request and transmit traffic instructions to vehicles. These documents were amended following publication of an Air Traffic Services Directive (ATSD-015) issued on 16 April 1984. This directive pointed out that some phraseology was imprecise and could give rise to misinterpretations when associated with ground vehicle movements. To remedy this situation, the directive stipulated that in future the word cleared would be eliminated from the phraseology dealing with vehicle movements at airports and would be replaced with an executive type of authorization. The personnel concerned in this occurrence knew the contents of these documents. Section 4.03 of TP 2633 stipulates that before proceeding onto manoeuvring areas on a controlled airport, the vehicle operator is to contact the ground controller for permission to proceed to a specific location by a specified route. Further, section 354.4 of the MANOPS stresses that air traffic personnel must be more vigilant during periods of restricted visibility to ensure that the runway is clear for use when required, and that they must remain aware of the location of ground traffic.